Was a solution found for this 'black attack'. I've got the same problem now. My LFS
has no idea so I've just started using Melafix.
I would strongly suggest you isolated the affected fish. I had with one a growth on its lip now I've got 8 black widows with the mystery 'black stuff'. They are in a tank full of different tetras but I don't see any point in isolating them now as 1 coloured tetra is showing signs of a black growth on his nose. I'm trying Melafix and will add Permafix at the end of the week if there is no change. After that, I'll try a suphur based white spot cure and after that ....................... heaven knows.
PS My affected black widows look exactly like the ones in your pics.